Error with to_matrix()

I read the stan functions reference, which has to_matrix (vector v, int m, int n)
But I got an error when doing the following:
to_matrix(Sigma[subject_starts[n]: subject_stops[n]], V[n], V[n]));
The error is Exception: to_matrix(matrix): rows * columns (36) and vector size (6) must match in size. However, I am pretty sure that my vector Sigma has size 36 and V[n] is 6 for the first loop. Does anyone know why I get this error? Thanks!

please share your code, or relevant sections.
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My covariance Sigma has different sizes for different subjects, so I am declaring it as a vector first, and use to_matrix() to calculate the marginal likelihood.

generated quantities{
  vector[cov_size] Sigma;

  for (n in 1:N){
    Sigma[cov_starts[n]: cov_stops[n]] = to_vector(B[subject_starts[n]: subject_stops[n]] * Theta * Theta' *  B[subject_starts[n]: subject_stops[n]]' + diag_matrix(rep_vector(sigma_eps^2,V[n])));
    log_lik_marg[n] = multi_normal_lpdf(Y[subject_starts[n]: subject_stops[n]] | zero_v[subject_starts[n]: subject_stops[n]], 
                                        to_matrix(Sigma[subject_starts[n]: subject_stops[n]], V[n], V[n]));

the expression Sigma[subject_starts[n]: subject_stops[n]] is picking out a slice of length 6, according to the error message.
Sigma may be size 36, but the slice isn’t, according to the math library.

Oh I see. Thank you so much, I forgot to change the index for Sigma in the to_matrix() function. Now it works!